With increasing communication traffic, a demand for a large-capacity optical network increases. A large-capacity optical network can be provided by increasing a baud rate or the multi-level degree of a modulation method. A baud rate is the number of times of digital data modulation per unit time. In a case where a baud rate is increased in quadrature multi-level modulation, signal transmission performance is degraded and a data reception error occurs on a receiving side in a state where a skew remains between an I (in-phase component) axis and a Q (quadrature component) axis of a transmitter. “In-phase/quadrature skew,” “I/Q skew,” or simply “skew” refers to the relative phases of the in-phase and quadrature (I/Q) components in optical coherent modulation formats. For the prevention of a reception error due to a skew, adjustment for minimizing a skew between the I axis and the Q axis is performed on a transmission side.
In general, in order to adjust a skew compensation amount, an evaluation value (Q value) is acquired on the receiving side while the setting value of a skew adjustment unit is changed on the transmission side and a skew adjustment value with which the best Q value is acquired is selected.
On the other hand, a configuration is known in which the pattern widths of data signals Pa and Pb generated from a pattern generator in an optical communication apparatus are reduced in stages and the amount of delay of a delay element is controlled until signal power Spower of a modulated optical signal is lower than or equal to a reference value Th in respective pattern widths (see, for example, International Publication Pamphlet No. WO 2012/093416).